Method of treating magnesium silicide alloys of aluminum



United States Patent 3,331,711 METHOD OF TREATING MAGNESIUM SILICIDE ALLOYS 0F ALUMINUM Auvo Iivar Kemppinen, Henrico County, Va., and Harris Albert Thomson, Phoenix, Ariz., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed Oct. 18, 1963, Ser. No. 317,158 9 Claims. (Cl. 148-117) This invention pertains to a method of treating a heattreatable aluminum alloy having magnesium silicide as a hardening constituent. In particular, it pertains to an improved technique for directly obtaining mechanical properties normally achieved only by a separate aging step subsequent to mechanical working, thus eliminating the necessity of subjecting the alloy to such aging treatment;

For many commercial applications it is necessary to provide aluminum alloys having temper properties designated as T6 in the Aluminum Association Temper Designation System. The T6 temper designation is normally applied to fabricated, i.e. mechanically worked products, which are solution heat-treated and then artificially aged. The conventional technique employed to obtain products having T6 temper properties entails material disadvantages. One disadvantage resides in the performing of the solution heat-treatment and aging operations on products in their fabricated form. Where products having non-uniform cross-sections are involved, such as non-symmetrical extrusions, difiiculty has been encountered in effecting uniform treatment of the product in all of its diverse dimensioned areas, and there is usually exhibited in such circumstances a tendency toward twisting and bowing, which sometimes cannot be corrected by stretching alone.

The multiple treatments heretofore required to obtain T6 temper properties have required extensive facilities for performing separate solution heat-treatment and aging operations and have entailed the time involved in separately performing these operations.

Practitioner in the art of aluminum metallurgy have long sought to develop improved techniques which eliminate heat treatments subsequent to mechanical working and to provide techniques which insure a more uniform alteration of the properties of an alloy being treated. However, such prior art attempts to produce improved treating techniques have themselves proven inadequate in many respects. For example, in one prior art technique, an aluminum alloy including magnesium and silicon is treated by being heated to a solid solution temperature and then cooled to a temperature Well above the aging temperature of the alloy. The partially cooled alloy is then worked and the formed article cooled. While improved temper properties are gained by this technique so as to eliminate certain heat treatments subsequent to Working, it is still necessary for the articles to be subsequently aged in order to develop the desired properties.

In certain prior art techniques applied to alloys other than magnesium silicide alloys of aluminum, it has been suggested that as the amount of cold work applied to an alloy is increased, less artificial aging may be required to produce maximum strength. Even with this suggestion, it has heretofore been considered necessary in commercial practices to include a substantial degree of aging subsequent to cold working, regardless of the degree of mechanical working.

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Another technique which has been applied specifically to aluminum alloys in the magnesium silicide series has entailed heating a billet to a solution heat treatment temperature, quenching the billet to a convenient extrusion temperature between 600 F. and l,000 F. (which is considerably higher than the normal aging temperature), extruding and quenching the billet, and then subjecting the billet to precipitation heat treatment so as to attain a T6 temper. Here again, the technique includes aging subsequent to mechanical working.

In recognition of the need for an improved and more efficient technique for treating magnesium silicide alloys of aluminum, it is an object of the present invention to provide such a technique which eliminates the necessity for aging treatment subsequent to mechanical working.

A further object of the invention is to provide a technique for obtaining desired temper properties, which technique is materially faster than previously known techniques and eliminates the necessity of providing separate aging facilities heretofore considered essential.

In accordance with the invention, an aluminum alloy including magnesium silicide as a hardening constituent may be treated so as to obtain improved physical properties by:

(1) Raising the temperature of the alloy to a solution heat treatment temperature and maintaining a solution heat treatment temperature for a sufiicient time to dissolve alloying constituents;

(2) Rapidly cooling the alloy to a temperature 'below the normal temperature range for spontaneous aging so as to retain the soluble alloying constituents in supersaturated solid solution; and

(3) Deforming the alloy to directly effect precipitation hardening thereof.

With this vastly simplified technique,at least TS and ordinarily T6 temper (hereinafter sometimes collectively referred to as substantially T6 properties) may be obtained without resorting to aging steps subsequent to the deforming step.

The manner in which this technique may be performed to accomplish the objects of the invention is demonstrated by the following examples pertaining to the treatment of 6061 and 6063 aluminum alloys. The published composition limits of those alloys appear below.

Silicon O.20-0.6 GAO-0.8. Ir 0.35max 0.7max.

0.10max 0.15-0.40.

Magnesium Chromium. Zinc- Titanium Others, each. Others, total Aluminum 0.15max. 0.15rnax. Balance Balance.

Example 1 mechanical properties of the extrusions were found to be in excess of minimum 6063-T6 requirements of 25,000 p.s.i. yield strength, 30,000 p.s.i. ultimate strength and 10% elongation. Items (a) and (b) of Table I show the results for extrusion ratios of 10 and 17.

Example 2 Additional tests of the type presented in Example 1 were conducted by extruding 6063 alloy specimens at an extrusion ratio of 30 with a Model 1800 Dynapak machine having a larger ram mass. The use of ambient temperature billets was found to produce properties satisfactory for the TS criteria but not T6, as shown in item (c) of Table I. An attempt was made to determine the temperature of the emerging extrusion as close to the die as possible, with the result that a measurement of about 690 F. was obtained. Since this was high enough to suggest the possibility of overaging, additional tests were made under otherwise the same conditions but using depressed temperature billets at about 105" F. and about 320 F. Under these conditions, T6 properties were substantially achieved (see items (d) and (e) of Table I).

temperature of said alloy in that range for sufficient time to dissolve the soluble alloying constituents; rapidly cooling said alloy to retain the dissolved constituents in supersaturated solid solution; and rapidly deforming the alloy to directly effect precipitation hardening thereof.

2. The method of claim 1, in which the solution heat treated alloy is deformed so intensively as to require prompt cooling to prevent overaging.

3. The method of claim 1, in which the solution heat treated alloy is deformed intensively to the extent of TABLE I Extrusion Billet Extrusion Cooling of U.T.S., 0.2% Elonga- Alloy Ratio 'lemp., Temp., Extrusion p.s.i. Y.S., tion,

F. F. p.s i Percent 10 Ambient 31, 700 21. 0 17 Ambient 36, 300 17. 0 Ambient 24, 400 26. 5 30 104. 6 27, 100 15. 5 30 320 33, 200 14. 0 13. 1 Ambient 35, 700 13. 5 l3. 1 97. 39, 600 12. 0

Example 3 raising its temperature appreciably, and overaging is pre- Reproducing the technique of Example 1 for 6061 alloy, with an extrusion ratio of about 13, provided the results indicated in Table I as item (f). Minimum T6 criteria for this. alloy are 35,000 p.s.i. yield strength, 38,000 p.si. ultimate strength and 10% elongation. In addition, the superior results shown in item (g) were produced with otherwise the same technique but using a depressed temperature billet (-97 F.) coupled with water quenching of the extrusion.

Our experience indicates, therefore, that any tendency toward overaging can be readily resisted by reducing the billet temperature or by quenching the extrusion, or by a combination of both; however, such controls have been found to be more important for treating 606 1 alloy than 6063. As a practical limit, it is considered that the alloys should not reach a temperature as high as 600 F. for a period of over 50 milliseconds. Lower maximum temperatures can be tolerated for greater periods of time, however, since the precipitation process is a function of both time and temperature. In view of the fact that precise measurement of the maximum temperature is diflicult, moreover, the practice of quenching the product is preferred, particularly where maximum mechanical properties are desired. This ordinarily suflices (without need for sub-ambient cooling of the alloy before working) to provide the desired extent of precipitation hardening.

The general class of alloys susceptible to treatment in accordance with the invention, of which 6061 and 6063 alloys are representative, have magnesium silicide as the principal hardening constituent. They are composed predominantly of aluminum but typically include up to about 1.5% magnesium and at least enough silicon to form a significant amount of the magnesium silicide hardening constituent. Wrought alloys of the 6000 series are included, as well as forging alloys which satisfy the aboveindicated compositional criteria and are ordinarily responsive to the same types'of conventional heat treatment practices.

While the present preferred practice of the invention vented by cooling said alloy to below ambient temperature prior to the deforming step.

4. The method of treating a heat-treatable magnesium silicide alloy of aluminum, comprising the steps of:

heating the alloy to a temperature in the range for solution heat treatment thereof and maintaining the temperature of said alloy in that range for sufiicient time to dissolve the soluble alloying constituents;

rapidly cooling the alloy below the temperature range in which spontaneous aging would otherwise be initiated, so as to retain the dissolved constituents in supersaturated solid solution; and

rapidly deforming the alloy to cause substantially complete precipitation hardening and directly effect therein substantially T6 properties. 5. The method of claim 4, in which the deforming step causes appreciable heating of the alloy, but is carried out rapidly enough to prevent overaging.

6. The method of extruding a heat-treatable magnesium silicide alloy of aluminum, so as to obtain without a separate aging step properties equivalent to -those ordinarily obtained by an aging treatment subsequent to extruding, comprising the steps of:

heating an extrudable billet of the alloy in the temperature range for solution heat treatment thereof, for suflicient time to dissolve the soluble alloying constituents, and rapidly cooling said billet to retain the dissolved constituents in sol-id solution;

introducing the billet into a high-rate. extrusion machine adapted for the cold extruding of said alloy, at a billet temperature below the range in which spontaneous aging of the alloy would otherwise be initiated in the unworked condition; and

rapidly extruding said billet to causesubstantially complete precipitation hardening of the alloy and directly effect substantially T6 properties in the resulting extruded shape.

7. The method of claim 6, wherein the billet is introduced at substantially ambient temperature.

5 8. The method of claim 6, wherein said billet is intro- 2,501,440 duced at a sub-ambient temperature in order to prevent ,34 ,512 overaging during said extruding. 3,031,299 9. The method of claim 6 comprising the further step 3,234,053

of preventing overaging subsequent to extruding by 5 quenching the resulting extruded shape as it emerges from the extrusion die.

6 3/1950 Dix 148-32.5 7/1958 Cooper 148-115 4/1962 Criner 14811.5 X 2/ 1966 Pryor 148-115 OTHER REFERENCES 'Dyn-apak: A New Dimension in High-Energy-Rate References Cited 551.

UNITED STATES PATENTS 2,063,022 12/1936 Beck 148l1.5

10 HYLAND BEZOT, Primary Examiner.

DAVID L. RECK, H. F. SAITO, Assistant Examiners. 

1. THE METHOD OF TREATING A HEAT-TREATABLE MAGNESIUM SILICIDE ALLOY OF ALUMINUM, COMPRISING THE STEPS OF: HEATING THE ALLOY TO A TEMPERATURE IN THE RANGE FOR SOLUTION HEAT TREATMENT THEREOF AND MAINTAINING THE TEMPERATURE OF SAID ALLOY IN THAT RANGE FOR SUFFICIENT TIME TO DISSOLVE THE SOLUBLE ALLOYING CONSTITUENTS; RAPIDLY COOLING SAID ALLOY TO RETAIN THE DISSOLVED CONSTITUENTS IN SUPERSATURATED SOLID SOLUTION; AND RAPIDLY DEFORMING THE ALLOY TO DIRECTLY EFFECT PRECIPITATION HARDENING THEREOF. 